The Sun and its halo.
The Sun and its halo.
Any vision, sight or ocular experience that could loosely be described as greyish is called an achromatic visual sensation. These sensations are described using words like grey, black, white, dark, bright, silvery, taupe, leaden, ecru, ashen, beige, pale and so on. The flag of the Malaysian state of Terengganu is used as an icon for achromatic visual sensations. You can click on icons to come back to a page like this for easy reference.

By the third hypothesis, the reference experience for achromatic sensation is seeing the Sun. So to make a binary description of an achromatic visual sensation, compare it to seeing the Sun. Report the result using one of the following algebraic statements. If the two experiences are not comparable, then say that the sensation is not an achromatic visual sensation and express this as


If the sensation is like seeing the Sun, then say that it is white. Express this as


If the sensation is not like seeing the Sun, then say that it is black and that


If it is both like and not-like seeing the Sun, then it is a composite sensation and

$-1 \le \delta_{w} \le 1$

The number $\delta_{w}$ is called the whiteness.

Adjective Definition
Whiteness $\delta_{w} \equiv \begin{cases} +1 &\sf{\text{if a sensation is white }} \\ -1 &\sf{\text{if a sensation is black }} \end{cases}$ 2-2
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